This invention relates to the treatment of coal ash to remove ammonia compounds that contaminate the ash as part of post-combustion treatments of exhaust gases to remove nitrogen compounds.
In 1990, the United States Environmental Protection Agency put into place the Clean Air Act Amendments which were designed to reduce the emissions of “greenhouse gases”. Among the emissions covered are the nitrogen compounds NO and NO2 referred to generically as NOx. NOx is generated through the combustion of coal and its generation is directly affected by combustion temperature, residency time and available oxygen. Several technologies have been developed to meet the mandated NOx reduction limits.
The NOx reduction technologies fall into two major categories. One category includes technologies that modify or control the combustion or firing characteristics. The effect of these approaches has been an increase in residual unburned carbon in the coal ash. The other category includes technologies that are employed after combustion has taken place. These technologies include selective non-catalytic reduction, selective catalytic reduction, and amine enhanced fuel lean gas reburn. These technologies involve adding ammonia (NH3), and a significant amount of the ammonia finds its way onto the coal ash, typically by combining with available sulphur and other compounds that attach to the ash particles.
Coal ash is a marketable product if it is not contaminated. The ash may be used, for example, in concrete products as a replacement for a portion of the cement. However, ash that has been treated to reduce NOx and which is contaminated either by unburned carbon or ammonia compounds is not marketable.
Systems have been developed which may be used to reduce the amount of ammonia compounds affixed to fly ash. For example, German Patent Application No. 3526756 describes a process for the reduction of ammonia residues from the treated fly ash of a steam generator. In this process, ammonia contaminated fly ash is treated with a hot treating gas, e.g., air heated to 700° C. to 800° C., so that the ammonia is driven off into the treating gas. In U.S. Pat. No. 4,911,900, there is disclosed an apparatus in which fly ash containing ammonium salts is recovered from the flue gas of a combustion chamber and then introduced into a fluidized bed apparatus in which ammonia is driven off from the fly ash. The stripping of ammonia from the fly ash is affected in the fluidized bed apparatus by direct heating with a hot gas at 700° C. to 800° C. Ammonia containing gas from the fluidized bed apparatus is then supplied to an ammonia scrubber from which an ammonia solution is removed for storage. While these systems may be suitable for certain installations, they do have efficiency problems, such as excessive heat loss, that may limit their use in cost sensitive installations.
Therefore, there is a need for an improved method and apparatus that can solve the unburned carbon and ammonia contamination problems in fly ash. More particularly, there is a need for an improved method and apparatus that can remove ammonia compounds from fly ash, and/or consume unburned carbon in fly ash to make the fly ash a marketable product.